Throughout this application, various publications are referenced by Arabic numerals within parenthesis. Full citations for these publications may be found at the end of the specification, immediately preceding the claims. The disclosures of these publications are hereby incorporated by reference into this application in order to more full describe the state of the art as known to one skilled therein as of the date of the invention described and claimed herein.
In a contact-dependent process termed "T cell helper function," CD4.sup.+ T lymphocytes direct the activation and differentiation of B lymphocytes and thereby regulate the humoral immune response by modulating the specificity, secretion and isotype-encoded functions of antibody molecules (1-8). The T cell surface molecules that mediate the contact-dependent elements of T cell helper function are not yet fully known (9).
The process by which T cells help B cells to differentiate has been divided into two distinct phases: the inductive and effector phases (10,11). In the inductive phase, resting T cells contact antigen-primed B cells and this association allows clonotypic T cell receptor (TCR) -CD4 complexes to interact with Ia/Ag complexes on B cells (5, 12-19). TCR/CD4 recognition of Ia/Ag results in the formation of stable T-B cognate pairs and bidirectional T and B cell activation (20-26). In the effector phase, activated T cells drive B cell differentiation by secreting lymphokines (27-30) and by contact-dependent stimuli (24,31-38), both of which are required for T cells to drive small, resting B cells to terminally differentiate into Ig secreting cells (31, 39-42).
Although the inductive phase of T cell help is Ag-dependent and MHC-restricted (5, 12-18, 40), the effector phase of T cell helper function can be Ag-independent and MHC-nonrestricted (31, 34, 36, 40, 43-50). An additional contrasting feature is that the inductive phase of T cell help often requires CD4 molecules and is inhibited by anti-CD4 mAb (19), whereas helper effector function does not require CD4 molecules (51) and is not inhibited by anti-CD4 mAbs (33, 34, 36, 49). The nonspecific helper effector function is believed to be focused on specific B cell targets by the localized nature of the T-B cell interactions with antigen specific, cognate pairs (25, 26, 52).
Although terminal B cell differentiation requires both contact- and lymphokine-mediated stimuli from T cells, intermediate stages of B cell differentiation can be induced by activated T cell surfaces in the absence of secreted factors (32, 33, 53-56). These intermediate effects on B cells include induction of surface CD23 expression (32, 35, 57), enzymes associated with cell cycle progression (37) and responsiveness to lymphokines (24, 37, 49, 54-56). Although the activation-induced T cell surface molecules that direct B cell activation have not been previously identified, functional studies have characterized some features of their induction and biochemistry. First, T cells acquire the ability to stimulate B cells 4-8 h following activation (38, 49). Second, the B cell stimulatory activity associated with the surfaces of activated T cells is preserved on paraformaldehyde fixed cells (24, 32, 37, 49, 56) and on purified membrane fragments (33, 53-55). Third, the B cell stimulatory activity is sensitive to protease treatment (24, 53, 54). Fourth, the process of acquiring these surface active structures following T cell activation is inhibited by cycloheximide (49, 54). Although these studies strongly suggest the existence of activation-induced T cell surface proteins that deliver contact dependent stimuli to B cells, the molecular identities of such structures have not previously been described.